backup.mjs script needs Mongo authentication

[rmunn]

Now that Mongo authentication has been deployed to staging and production, the backup.mjs script no longer works, because I had it sending Mongo queries without auth. I need to add the ability to take authentication parameters from environment variables, and/or possibly from command line args. I'll start with env vars as command line args will be a more extensive rewrite (currently the script only takes positional parameters, which is easy — but taking command line args will require adding a parser library, because there are too many corner cases such as -- to do the parsing by hand).